Communication system



May i7, 1938.

W. P. PLACE 2,117,358

COMMUNICATION SYSTEM F'iled Feb. 6, 1934 Toeopz'vin/:M Appapalus T0Tpanfmz'zzmg Appapafar SM' 55K-TAT Flle!" 4M 1 INVENTOR 15 WillardRPZCZC@` BY @X1/JNM HIS ATTQRNEY Patented May 17, 1938 UNITED STATESPATENT OFFICE COMMUNICATION SYSTEM Application February 6, 1934, SerialNo. '709,971

7 Claims.

My invention relates to communication systems, and particularly toycarrier telephone systems using suppressed carrier transmission.

An object of my invention is the provision, in

systems of the type referred to, of an improved circuit arrangement inconjunction With anv electron tube modulator-oscillator for suppressionof the carrier which is modulated b-y the voice frequencies. Anotherobject of my invention is`the provision of a similar circuit arrangementin conjunction with an electron tube oscillator for suppression of thereintroduced carrier which is mixed with the incoming sideband fordemodulation. Other objects and advantages of my invention will appearas the specication progresses.

In carrier telephone systems using suppressed carrier transmissionheretofore proposed, the combination resulting from modulation of thecarrier by voice frequencies is applied to a selec'- tive network ofbalanced circuits and the carrier and one sideband are substantiallysuppressed and the remaining sideband is passed t0 the communicationcircuit. The carrier being of irelatively large magnitude, itssuppression is `,more difcult and necessitates a relatively elaboratelter arrangement. A balanced modulatoroscillator consisting of twoVacuum tubes operating in the conventional push-pull manner has alsobeen proposed for carrier telephone systems for balancing out thecarrier but such arrangement requires two Vacuum tubes. In my presentinvention a bridge type circuit arrangement for an electron tubefunctioning in conjunction with a tuned circuit for generating a carriercurrent which is to be modulated by voice frequencies is provided. Thisbridge circuit substantially eliminates the carrier from the output ofthe tube by confining the carrier to the bridge circuit, and theassociated filter needs only to select between the upper and the lowersideb-and frequencies produced from modulation with the result that theiilter network can be of comparatively simple construction. At thereceiving location a similar bridge type circuit for the tuned circuitand the electron tube which generate the carrier to be mixed with theincoming sideband for demodulation is provided. This last mentionedbridge circuit operates to substantially eliminate the reintroducedcarrier from the receiving apparatus by coniining the carrier to thebridge circuit.

I will describe one form of apparatus embodying my invention, and willthen point out the novel features thereof in claims.

In the accompanying drawing, Figs. 1 and 2, when taken togetherconstitute a diagrammatic (Cl. Z50-6) viewof one form of apparatusembodying my invention, and which apparatus is particularly adapted fortelephone communication between two locations on a railway train. Theapparatus of Fig. 1 is that for transmitting, and the apparatus of Fig.2 is that for receiving. It will be understood that in practicing myinvention both locations will be equipped with the transmittingapparatus of Fig. 1 and the receiving apparatus of Fig. 2 so thattwo-way operation may be accomplished. Fig. 3 is a diagram illustratingthe i bridge type circuit arrangement of Figs. 1 and 2.

Although the apparatus of Figs. 1 and 2 may be located at any twopoints: on a railway train, they will be referred to in the followingdescription as being mounted on the locomotive and in the Caboose of afreight train, respectively. As stated above, in actual practice, boththe locomotive and the Caboose will be equipped with the apparatus ofFigs. 1 and 2 in order to provide for two-way communication. Thecommunication circuit from one end of the train to the `other may takedifferent forms, and preferably isthat disclosed in the L. O. Grondahlapplication for Letters Patent of the United States Serial No. 450,135,led May 6, 1930, for Electric train signaling systems, and wherewithtransmission is accomplished between the two ends of a `train throughthemedium of the traffic rails.

Although `a specific adaptation of my invention is here disclosed, itwill be understood that I do not wish to limit myself to telephonesystems for railway trains, and my invention is equally useful to othertelephone systems of the type here involved. The form here disclosedwill serve,

however, to illustrate the many applications of my invention. As willappear hereinafter, my invention is not limited to telephonecommunication and may be utilized for the transmission of code messagesor for the transmission of control impulses.

Referring to Fig. 1, the reference character MO designates apparatus forgenerating carrier current of any convenient frequency say, for example,'2000 `cycles per second and which is to be modulated by voicefrequencies. The apparatus MO comprises an electron tube and a tunedcircuit associated therewith by a bridge type circuit arrangement. Theelectron tube 5 may take different forms and preferably is a pentodehaving a filament 6, a plate l, a screen grid 8 and a control grid 9.The lament 6 is heated by a battery l0 in the customary manner and isprovided with a cathode grid ll interposed between the screen grid 8 andthe plate 1 to shield the screen grid from the secondary electronemission from the plate. A tuned circuit I9 consisting of an inductor I2and a condenser I5 is associated with the tube by having one terminal,the righthand terminal as shown in Fig. 1, connected with the plate 'Iover a wire I6 and its opposite or lefthand terminal connected with thescreen grid 8 over a wire I 1. A suitable source of current here shownas a battery I3 is connected between a mid terminal I8 of the inductorI2 and the lament 6, the primary winding I4 of a transformer TI beinginterposed between the mid terminal I8 and the battery I3. The parts areso proportioned that substantially an equal voltage is applied to theplate I and the screen grid 8. The tube 5 is preferably of the pentodetype which functions properly with equal voltage applied to the plateand screen grid, such tubes being wellknown to the art. The parts arefurther so proportioned that oscillations take place between the plate land the screen grid 8 through the tuned circuit I9 which oscillations,as stated above, are of a frequency of '7000 cycles per second. It willbe understood, of course, that other frequencies could be generated andthat the frequency of '7000 cycles per second is assumed by way ofillustration only. It is clear that there is current flowing between theplate 'i and the filament 6 and a similar current iiowing between thescreen grid 8 and the lament 8, tube 5 being preferably of suchcharacteristics that the alternating current components of thesecurrents are of approximately equal magnitude in response toapproximately equal voltages impressed upon the plate and screen grid.It will be understood, however, that pentode tubes havingcharacteristics other 'than that which produces a substantially equal owof current as the result of equal voltage on the plate and screen gridmay be employed if found desirable, the mid terminal I 8 of the inductorI2 being so adjusted as to produce a balanced condition. Theseconductances of the tube 5 may be represented by resistors Rp and Rgconnected between the filament 6 and plate 1, and between the filament Sand grid 8, respectively. To visualize these internal impedances of thetube 5, the ctitious resistors Rp and Rg are shown on the drawing bydotted lines. However, since the internal impedances of the tube 5 willordinarily vary somewhat with voltage and different individual tubes,actual resistors Rp and Rg may be connected in the circuits at thelocations shown if found desirable to do so in order to obtain greaterstability.

It is at once apparent that this arrangement between the tube 5 and thetuned circuit Iii forms a bridge circuit, the portions of the inductorI2 n either side of the mid terminal I8 forming two arms of the bridge,and the plate and grid resistors Rp and Rg forming the other two arms ofthe bridge. In Fig. 3 this bridge circuit is shown diagrammatically thatit may be more clearly understood. When oscillations are generatedbetween the plate and the screen grid, the only part of the carriercurrent flowing in the connection from the mid terminal I8 to thefilament 6 is a small part that may be caused by an unbalanced conditionof the bridge circuit.

This carrier frequency current is modulated by voice frequencies in thefollowing manner. The control grid 9 of the tube 5 is provided with acircuit including the secondary winding 20 of a transformer T2 and abattery 2|. The primary winding 22 of the transformer T2 is included ina circuit which also contains a battery 23 and a telephone transmitteror microphone M. Hence, vibrations produced by sound impulses in themicrophone M cause corresponding variations in the current flowing inthe primary winding 22 which variations induce an electromotive force ofa corresponding frequency in the secondary winding 20 and thiselectromotive force, in turn, is impressed between the control grid 9and the filament 6 of the tube 5. With. the instantaneous potential ofthe control grid B with respect to the filament E varied at therelatively low voice frequencies produced by sound impulses in themicrophone M, the amplitude in the oscillations taking place in thetuned circuit I9 is varied accordingly. It follows, therefore, that thecarrier frequency is modulated in accordance with the voice frequenciesand, consequently, upper and lower sideband frequencies are produced.

Since most of the carrier is confined to the bridge circuit, and sincethe voice frequencies are not aiected by the bridge circuit, the currentflowing through the primary winding I 4 of the transformer TI chieflyconsists of voice frequencies and the upper and the lower sidebandfrequencies. It follows that the electromotive force induced in thesecondary winding 24 of the transformer TI chiefly consists of thesideband frequencies and Voice frequencies, only the small unbalancedportion of the carrier being carried over. This electromotive forceinduced in the secondary winding 24 is conveyed to a band-pass filterBPF so proportioned and adjusted as to pass one sideband with verylittle attenuation and to substantially suppress all other frequencies,the upper sideband being preferably passed.

The filter BPF may take different forms several of which are well-knownto the art and as its specific structure forms no part of my invention,it is shown conventionally only in order to simplify the drawing. It isclear from the foregoing description, however, that the magnitude of thecurrent applied to the band-pass filter BPF will be relatively small,the major portion of the carrier being eliminated by the bridge circuit,and hence this filter may be relatively simple in construction.

The upper sideband frequency passed by the filter BPF is supplied to thecommunication circuit, in this instance being rst amplied to arelatively high energy level. As here shown, the sideband passed by thelter BPF is impressed between the grid 25 and the filament 26 of anelectron tube amplifier 2l with the result that the upper sidebandfrequencies are reproduced in the plate circuit of the tube 21 greatlyincreased in amplitude. The plate circuit for the tube 2l includes theinput side of an amplifier transmitter AT shown conventionally only andpreferably consisting of two electron tubes connected to operate in thewell-known push-pull manner, and, hence the upper sideband frequenciesare further amplied to a relatively high energy level. The output sideof the amplifier transmitter AT includes the primary winding 28 of anoutput transformer T3, the secondary winding 29 of which is adapted tobe coupled with the communication circuit through the medium of a pairof inductor coils 3U and 3|.

The inductor coils 3U and 3l are mounted on the locomotive in inductiverelation with the trafc rails I and Ia, respectively. Normally theseinductoi coils will be connected with the receiving apparatus on thelocomotive to be described later over normally closed contacts 5I and 52of acircuit controlling selector SW. Reversing this circuit controllingselector SW to the position indicated by the dotted lines in Fig. 1,transfers the coils 30 and 3| from the receiving apparatus to thesecondary winding 29 of the output transformer T3, the circuit beingcompleted over the contacts 56 and 5l. In the form here shown thecircuit controlling selector SW is manually operated. It will beunderstood, however, that the selector SW may be automatically operatedby suitable coding apparatus and the coils 30 and 3! alternatelyconnected with the receiving and transmitting apparatus should it seemdesirable to do so.

Referring to Fig.`2, 33 and 34 are inductor coils mounted on the caboosein inductive relation with the traffic rails l and la, respectively.These coils 30 and 3i are connected with an input filter Fl overnormally closed contacts 35 and 36 of a manually operated circuitcontrolling switch SWI The filter FI is preferably a band-pass filter,the parts of which are so proportioned and adjusted as to pass a band offrequencies. having a width substantially equal to the voice frequencyrange and to attenuate all other frequencies. In this instance thefilter Fl is so proportioned as to pass the upper sideband of thecarrier of the apparatus of Fig. 1 modulated by the voice frequencies.That is, the filter Fl passes a band extending from 7500 to 9500 cycles,the carrier having been assumed to be I7000 cycles per second and thevoice frequencies being taken as extending from 500 to 2500 cycles. Thisupper sideband passed by the lter Fl is applied to the input of anamplifier AM and is made to appear in the output circuit of thatamplifier greatly increased in amplitude. The iilter Fl and amplifier AMmay each be any one of several well-known types for such devices and asthe specific type of each forms no part of my present invention, theyare shown conventionally only for the sake of simplicity. The output ofthe amplifier AM includes the primary winding 3l of a couplingtransformer T4 and hence an electromotive force having a frequencycorresponding to the sideband picked up from the traiiic rails isinduced in the secondary winding 38 of that transformer.

The receiving apparatus. of Fig. 2 is provide with a demodulator D whichincludes a pentode 45 and an associated tuned circuit 49 both preferablysimilar to corresponding devices of the modulator oscillator MO of Fig.l. The bridge circuit arrangement by which the tuned circuit 49 of Fig.2 is connected with the plate 41 and screen grid 48 of the pentode 45 isidentical to that described for the tuned circuit I9 and pentode of Fig.l and it is thought not necessary to repeat the description. Suffice itto say that the parts are so proportioned and adjusted that oscillationsof the carrier frequency of '7000 cycles per second are generated and owin the bridge circuit, the small unbalanced portion of the carrier onlyflowing in the primary winding 44 of a transformer T5 interposed betweenthe mid terminal 50 of the inductor 42 and the battery 43. It is clearthat this carrier will be mixed with the sideband frequencies induced inthe secondary Winding 33 of the coupling transformer T4 and applied tothe control grid 49, and that there will be produced corresponding upperand lower sidebands. The lower sideband will consist of the receivedupper sideband less r1000 cycles, name- 1y, a band ranging from 500 to2500 cycles, it being recalled that the upper sideband consists of '7500to 9500 cycles. In other words, this lower sideband will correspond tovoice frequencies.

The upper sideband resulting from the mixing of the local carrier withthe received upper sideband frequencies will consist of the receivedsideband plus 7000 cycles. The combination produced by the mixing of thelocal carrier with the received sideband will appear in the primarywinding 44 of the transformer T5 and will induce an electromotive forceof corresponding frequencies in the secondary winding 53 of thattransformer. The secondary winding 53 is connected with the input sideof a band-pass lter F3 having parts so proportioned and adjusted as topass the voice frequencies and to substantially suppress the remainingproducts of demodulation. The voice frequencies passed by the filter F3are then applied to a telephone receiver here shown as a loud-speakerLS, after having been first amplied by an amplier AMI of the usual typeand whose output circuit includes the primary winding 54 of atransformer T6, the secondary winding 55 of which is connected with theoperating winding of the loud-speaker LS.

To describe the operation of my system as a whole I shall assume thatthe circuit controlling switches SW and SW! occupy the positionillustrated on the drawing with the result that the receiving apparatuson the locomotive and the caboose are both coupled with thecommunication circuit. As stated hereinbefore, both locations will beprovided with the transmitting apparatus and the receiving apparatus ofFigs. 1 and 2, respectively. To telephone from the locomotive to thecaboose the operator on the locomotive will reverse the circuitcontrolling switch SW and speak into the microphone M. The upper sideband is then supplied to the traiiic rails through the medium of theinductor coils 30 and 3l having rst been amplified, however, to arelatively high energy level. The transmitted upper side bandfrequencies will be picked up from the traffic rails at the caboose bythe inductor coils 33 and 34 and impressed upon the filter FI since theswitch SW is now in the position for connecting the coils 33 and 34 tothey receiving apparatus. After amplification the received sidebandfrequencies are demodulated and the resulting voice frequencies aresupplied to the caboose loudspeaker. To permit conversation from thecaboose to the locomotive the caboose operator will shift the circuitcontrolling switch SWI to the position for connecting the inductor coils33 and 30.! with the transmitting apparatus on the caboose and thelocomotive operator will restore the circuit controller SW to theposition illustrated for connecting the receiving apparatus on thelocomotive with the inductor coils 30 and 3|. From this point on theoperation will be similar to that already described in telephoning fromthe locomotive to the caboose.

While a telephone system has been described, it is clear that byreplacing the microphone M with a generator adapted io supply apredetermined frequency within the voice frequency range, say forexample, a frequency of 1000 cycles per second, and then coding thiscurrent according to a prearranged code, the loud-speaker at the otherlocation will sound a note corresponding to the prearranged code andcode messages can be communicated between the two locations.Furtherinore, by replacing the loud-speaker LS with tuned circuitnetworks selectively responsive to different low frequencies within thevoice frequency range, an automatic control can be established betweenthe two locations Although I have herein shown and `described only oneform of apparatus embodying my invention, it is understood that variouschanges and modications may be made therein within the scope of theappended claims without departing from the spirit and scope of myinvention.

Having thus described my invention, what I claim is:

l. In combination; an electron tube provided with a cathode, a plate andtwo grids; a tuned circuit having opposite terminals connected onedirectly to said plate and the other directly to one of said grids andsaid circuit including an inductor, circuit means for connecting amid-terminal of said inductor with said cathode, a current sourcedisposed in said circuit means to supply substantially equal voltages tosaid plate and said one grid for causing oscillations of a predeterminedcarrier frequency to be generated by the tuned circuit and balanced outof said circuit means, means for varying the potential of the other gridof the tube with respect to the cathode at a given frequency formodulating said oscillations wheieby side bands corresponding to saidcairicr modulated at said given frequency flow in said circuit meanswith the carrier frequency substantially eliminated, and means coupledwith said circuit means for receiving said side bands.

2. In combination; an electron tube provided with two grids as well as aplate and a cathode, a tuned circuit having opposite terminals connectedone directly to said plate and the other directly to one of said gridsand said circuit including an inductor, a circuit connection forconnecting a mid-terminal of said inductor with the cathode to form abridge circuit consisting of the two portions of said inductor and thetube impedances existing between the plate and the cathode and betweensaid one grid and the cathode, a source of current interposed in saidcircuit connection to supply'voltages to the plate and said one grid forcausing oscillations of a predetermined carrier frequency between theplate and said one grid through the tuned circuit and which are balancedout by said bridge circuit, means for impressing between the other gridand the cathode a side band telephone current whereby side bandfrequencies resulting from demodulation ow in said circuit connectionwith the carrier frequency substantially eliminated, receiving apparatusincluding a loud-speaker and a filter adapted to pass voice`frequencies, and a transformer having one winding interposed in saidcircuit connection and a second winding connected with the inputterminals of the filter of the receiving apparatus.

3. In combination with telephone receiving apparatus including a rstfilter adapted to pass a given side band of a predetermined carrierfrequency modulated with voice frequencies, a second filter adapted topass voice frequencies and a loud-speaker connected with the output sideof said second filter; an electron tube having two grids as well as aplate and a cathode, a tuned circuit including an inductor and acondenser and tuned to resonance at said predetermined carrierfrequency, means to connect one terminal of said tuned circuit directlyto the plate and a second terminal of said circuit directly to one ofthe grids of said tube, means including a source of current forconnecting a mid-terminal of said inductor with the cathode of said tubefor causing the tuned circuit to produce oscillations of said carrierfrequency and for suppressing said carrier frequency in the connectionbetween said midterminal and cathode, means to connect the output sideof said first lter between the other grid and cathode of the tube, atransformer having one winding interposed in the connection between saidmid-terminal and the cathode anda second winding connected across theinput terminals of said second filter, and means for at times connectingthe input terminals of the rst lter with a communication circuit.

4. In combination, a communication circuit, a lter adapted to pass agiven side band of a carrier of predetermined frequency modulated with asignaling frequency, means to connect at times the input terminals ofsaid lter with said circuit; an electron tube provided with a cathode, aplate and two grids; a tuned circuit including an inductor and acondenser and tuned to resonance at said predetermined carrierfrequency, means to connect one terminal of said tuned circuit directlyto said plate and a second terminal directly to one of said grids, othermeans including asource of current for connecting a mid-terminal of saidinductor with said cathode for causing said tunedcircuit to produceoscillations of said carrier frequency and for suppressing said carrierfrequency in the connection between said mid-terminal and cathode, atransformer having one winding interposed in the connection between saidmid-terminal and cathode, receiving means including a signaling deviceresponsive to said signaling frequency and a filter adapted to pass suchfrequency and con nected to a second winding of said transformer, andmeans to connect the output terminals of the rst mentioned filter withthe other grid and cathode of said tube.

5. In combination; an electron tube provided with a cathode, a plate andtwo grids; a tuned circuit having one terminal connected directly tosaid plate and a second terminal connected directly to one of said gridsand including an inductor, circuit means for connecting a mid-terminalof said inductor with said cathode to form a bridge circuit consistingof the two portions of said inductor and the tube impedancesexistingbetween said plate and the cathode and between said one grid andthe cathode, a source of current interposed in said circuit means tosupply substantially equal direct current voltages to said plate andsaid one grid for causing oscillations to take place between the plateand said one grid through the tuned circuit, and means for impressingupon the other grid of the tube the frequencies of a signaling currentto vary the amplitude of said oscillations as a function of theinstantaneous value of the signaling current to cause current of theresultant side band frequencies with the carrier suppressed to flowbetween said mid-terminal and said cathode.

6. In combination; an electron tube provided with a cathode, a plate andtwo grids; a tuned circuit having its opposite terminals connected onedirectly to the plate and the other directly to one of said grids andincluding an inductor, circuit means for connecting a mid-terminal ofsaid inductor with said cathode, a current source disposed in saidcircuit means to supply voltage to said plate and said one grid forcausing the tuned circuit to generate a predetermined carrier frequencycurrent, means for varying the potential of the other grid of the tubewith respect to the cathode at the frequency of a signaling current tovary the amplitude of the carrier current as a function of theinstantaneous value of the signaling current to produce in said circuitmeans the resultant side band frequencies with the carrier frequencyeliminated, and means coupled With said circuit means for receiving saidside bands.

'7. In combination; an electron tube provided With a cathode, a plateand two grids; a tuned circuit having its opposite terminals connectedone directly to said plate and the other directly to one of said gridsand including an inductor, circuit means for connecting a mid-terminalof said inductor with the cathode to form a bridge circuit consisting ofthe two portions of said inductor and the tube impedances existingbetween the plate and the cathode and between said one grid and thecathode, a source of current interposed in said circuit means to supplyvoltage to the plate and said one grid for causing oscillations of apredetermined carrier frequency between the plate and said one gridthrough the tuned circuit, means for varying the potential of the othergrid of the tube with respect to the cathode at a given frequency tovary the amplitude of said oscillations as a function of theinstantaneous value of said given frequency to produce in said circuitmeans the resultant side band frequencies with the carrier frequencysubstantially eliminated, and means coupled with said circuit means forreceiving said side bands.

WILLARD P. PLACE.

